Nutrient Transport Modelling in the Daugava River Basin - DiVA Portal

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3.2 MODEL DESCRIPTION3.2.1 An overview of the modelThe GWLF model simulates monthly stream flow and monthly loads of phosphorus andnitrogen from any watershed. Hydrology is the driving force of the model. There arethree types of water storage; soil water (unsaturated zone), shallow groundwater(groundwater box 1) and deeper groundwater (groundwater box 2). The amount ofwater in these three storages characterises the system at each moment and determinesthe evapotranspiration, surface runoff and groundwater flows. The catchment is dividedinto land use categories with different characteristics (different parameter values).Surface runoff from each land use area and groundwater flow from the entire catchmentto the river are predicted from data on daily precipitation and temperature. The user hasto state the nutrient concentration in surface runoff, groundwater and in eroded material.The nutrient load is obtained by multiplication of the nutrient concentration by the waterflow. Point sources of nutrients are added in order to obtain the total nutrient load. Inthe original GWLF model the nutrient load from septic systems in the catchment isincluded. This part of the model has been excluded in the study due to difficulties withfinding data on the number of people that have different kinds of septic systems. Figure4 shows a schematic picture of the model.EvapotranspirationPrecipitationLand usecharacteristicsTemperatureInput dataInfiltrationSnowmeltFunctionSoil water(unsaturatedzone)Nutr.conc.inerodedmaterialErosionSurfacerunoffNutr. conc. insurface runoff(land usespecific)Water storagePercolationOutputNutr. conc. inbox 1Groundwaterbox 1r1PointsourcesPercolationNutr. conc. inbox 2Groundwaterbox 2r2StreamchannelStreamflowNutrient loadFigure 4. The structure of the modified GWLF model used in this study.The model calculates flows and nutrient loads for each land use and the contributionsfrom the different land uses are summed up in order to obtain the total flow/load fromthe watershed. In Appendix A there is a list of all the input data and parameter valuesused in this study.8
3.2.2 Water flowThe structure of the hydrological part of the GWLF is shown in figure 1. The inputneeded for this part of the model is data on precipitation and temperature as well as theareas of each land use. Streamflow is computed as the sum of groundwater flow andsurface runoff, as shown in equation 1. All the flows are divided by the watershed areawhich is how the unit centimetre is obtained.Streamflow t = Q t + G1 t + G2 t (1)whereStreamflow t = streamflow on day t [cm]Q t = surface runoff to the stream channel on day t [cm]G1 t = flow from groundwater box 1 to the stream channel on day t [cm]G2 t = flow from groundwater box 2 to the stream channel on day t [cm]Surface runoff is computed from weather data by the U.S. Soil Conservation Service’sCurve Number Equation. This is further described in Appendix B.Water that does not contribute to surface runoff will infiltrate the soil. Infiltrated waterwill then either be lost to the atmosphere through evapotranspiration or percolate to thegroundwater zone. Evapotranspiration is limited by available soil moisture in theunsaturated zone and by potential evapotranspiration:Et= Min CV ⋅ PE ; U + R + M − Q )(2)(t t t t t twhereCV t = evapotranspiration cover coefficientPE t = potential evapotranspiration [cm]U t = unsaturated zone moisture [cm]R t = rainfall on day t [cm]M t = snowmelt on day t [cm]= surface runoff from land use k on day t [cm]Q tThe evapotranspiration cover cover coefficient CV t is dependent on land use andseason. It has a higher value during the growing season since the transpiration of thevegetation is then greater. CV t values for different land uses and for dormant season andgrowing season respectively are given in The BasinSim 1.0 User’s Guide (Dai et al,2000). Potential evapotranspiration for each day is calculated from number of daylighthours, saturated water vapour pressure and daily mean air temperature. The equationsfor determining potential evapotranspiration are given in Appendix C.Percolation is assumed to occur each day, so that every day all the unsaturated zonemoisture percolates into the saturated zone. It is thus assumed that the unsaturated zoneis completely emptied each day. If there is no moisture in the unsaturated zone, therewill be no percolation:PC t = Max(0 ; U t +R t +M t Q t E t ) (3)9
Page 1 and 2: UPTEC W05 009Examensarbete 20 pFebr
Page 3 and 4: Nyckelord: avrinning, näringsämne
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Page 8 and 9: simulation system CatchmentSim. The
Page 10 and 11: Figure 1. The hydrological part of
Page 12 and 13: TemperaturePrecipitation20.00120deg
Page 16 and 17: Groundwater discharge to the river
Page 18 and 19: Groundwater nutrient loadsThe groun
Page 20 and 21: There is therefore no guarantee tha
Page 22 and 23: a) Streamflow, calibration periodb)
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Page 26 and 27: the chosen value is a very rough ap
Page 28 and 29: all cultivated land will have winte
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Page 34 and 35: Description of parameter/input Name
Page 36 and 37: Description of parameter/input Name
Page 38 and 39: moisture.Figure B1. Selection of cu
Page 40 and 41: wherea t = rainfall erosivity coeff
Page 42 and 43: Appendix F. Regression lines of rep

Nutrient Transport Modelling in the Daugava River Basin - DiVA Portal

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